Videos are definitely important. I was thinking however of written accounts by observers of these sessions. Several have appeared in DanceView in recent years, I believe. (Unfortunately I recently gave away my archives of DV and similar serious publications, and cannot recollect the specific works.)
There is a video of Verdy coaching dancers of the Paris Opera Ballet--it's quite amazing and, I believe, commercially available (edited to add: but it's of Liebeslieder, not Emeralds). But Bart is right about the importance of written accounts. I would add that I think more extensive scholarly work should also be undertaken to capture all of these voices in a systematic way.